Disposal of sludge produced as a waste product in wastewater treatment has been and continues to be a worsening problem which has had widespread and intensive attention for more than twenty years. Currently, the annual production of sludge by municipal sewage treatment plants is estimated to be in excess of seven million tons (dry basis) per year. Thus, for example, a single plant in Carson, Calif., serving a portion of the Los Angeles County area produces two hundred tons per day. It is estimated that by the year 2000, a Los Angeles, Calif. City plant located at Hyperion, Calif., will be producing over 400 tons per day of primary and activated sludges. Volumes of sludge produced are rapidly increasing, while available options for disposal are restricted or decreasing.
Principal among existing disposal methods are ocean dumping, land fill dumping, land application (e.g., to agricultural, forested and strip-mined land), and incineration. Ocean dumping is restricted by Federal legislation, and the number of incineration plants has been decreasing in recent years due in part to tightening Federal air quality regulations. Continuing concern over potential effects of transmission of the heavy metal and toxic chemical components from sewage sludges and other sludges into the food chain and ground water has restrained the growth of land application and increasingly restricted the number of landfill sites which may be used for sludge disposal. Nevertheless, as available landfill sites fill up and diminish in number, Federal policy has turned increasingly in the direction of encouraging land application with appropriate controls.
For many years it has been recognized that reduction of the water content of sludge is of great importance in implementing the majority of these disposal techniques. Excessive water content escalates incineration costs, unnecessarily increases the proportion of precious landfill space occupied by a given weight of sludge, significantly increases the number of trucks that must be used to move a given weight of sludge solids to a disposal site, limits the distance over which transport of sludge to agricultural and other land application sites is economically feasible, and must be eliminated in preparing sludge for some of its more constructive end uses. Also, it has long been recognized that removal of water from sludge reduces both its weight and volume. For example, the drying of sludge from a 90% moisture content to a 60% moisture content results in reducing the sludge weight and volume to only one-fourth of the starting amounts. This simple mathematical fact, coupled with the unruly behavior of sludge during drying, has spurred large numbers of talented engineers and researchers to devote continuing and careful attention to the development of better and more economical sludge drying equipment and techniques.
Not the least among its defiant characteristics is the slimy, gelatinous or paste-like character exhibited by many forms of sludge. Pasty sludge material tends to stick to almost everything it touches. When a layer of sticky sludge contacts the hot walls or other internal surface of a drying device, there is a tendency for it to stick tenaciously to the surfaces and to grow in tenacity as the drying process reduces its moisture content. This tenacious layer provides a foundation for the accretion of additional thicknesses of dried sludge which act as thermal insulation. Thus, when the dryer surfaces, which become coated in this manner, are the surface through which heat is intended to be transmitted to the wet sludge, drying efficiency plummets and/or the equipment must be shut down frequently for cleaning.
Throughout a lengthy search, a wide variety of systems have been proposed and used, often with unsatisfactory results or complete failure. It is notable that the various types of equipment, which have failed in sludge drying, have generally been excellent units which performed admirably in the drying of a wide variety of other materials. Among the devices proposed and used for sludge drying are: hollow disk (e.g., Envirotech Thermo-Disc (.TM.) dryers, which are understood to have experienced failures due to excessive erosion and corrosion when used to dry sewage sludge; thin film dryers (e.g., Luwa (.TM.)), which have apparently been used with some success with sludges dispersed in the form of fluent or readily flowable slurries; rotary, scraped drum dryers, which have a long history of successful operation with fluent slurries, but which tend to be thermally inefficient when applied to sewage sludges; and flash dryers (e.g., Raymond (.TM.)) which are understood not to have worked successfully on gelatinous wet sludges. Apparently, multi-hearth furnaces have been used at Harrisburg, Pa. and elsewhere, but the city of Harrisburg was not fully satisfied since they and their contractor, Bethlehem Steel Corporation, have engaged in a seven-year, unsuccessful effort to replace these furnaces with Bethlehem Porcupine (.TM.) internally heated rotary cut-flight type dryers in the drying of sewage sludge wetted primarily with water. Tray/shelf (e.g., Wyssmont Turbo (.TM.)) dryers have been used with some success in this application in a number of locations and are presently understood to provide the best combination of capacity, efficiency and maintenance cost currently available.
Another type of equipment which skilled workers have attempted to adapt to sludge drying is the rotary heated screw dryer. Prior workers have been attracted to this type of dryer because, in theory, it should, if it could be made to work successfully with sludge, offer more compact, sturdier installations, lower maintenance and operating costs and less difficulties with overloading than Wyssmont dryers and multi-hearth furnaces, and considerably greater thermal efficiency than the rotary drum dryers. Indeed, Perry, Chemical engineers' Handbook, McGraw-Hill Book Company, 1950, page 862, suggests drying of pasty materials with internally heated screw type dryers, and indicates that ". . . recycling of the dry product into the feed may be required to permit suitable handling in the dryer." This same work states in Table 33 on pages 872-873 that such dryers "[c]an only be used if material does not stick or cake."
In confirmation of the foregoing, the Denver division of Joy Manufacturing Company made a determined effort for years, now abandoned, to apply its highly developed and otherwise widely successful Holo-Flite (.TM.) heated rotary screw processors to the drying of sludge. It is understood that in their above-mentioned abortive attempt to dry aqueous sewage sludge with the Porcupine (.TM.) cut-flight dryers at Harrisburg, Pa., Bethlehem could not operate at recycle ratios of 3:1 and eventually experimented with recycle ratios as high as 5:1 before abandoning the project. Over a period spanning more than thirteen years, one of the present inventors, H. J. Buttner, experimented with the heated screw sludge dryers described in his U. S. Pat. Nos. 3,775,041 and 4,371,032.
Considering the heavy fouling of the heated screws that had occurred in the prior screw type devices, it seemed evident that the correct path to success in overcoming these problems would be to equip the screws with cleaning devices. This was the main thrust of the early work of Mr. Buttner, as reflected in the teachings of his above-mentioned patents. These described, respectively the cleaning of the screw flights (somewhat analogous to screw threads) and of the screw volutes (helical valleys between the flights) using recirculating balls or continuous loop scraper mechanisms having scrapers, either of which rode along in the dryer volutes with the drying sludge as the screws rotated. Unfortunately, the recirculating ball unit suffered from excessive torque build-up, leading to experimentation with the continuous loop scrapers. These kept the screws clean, but imposed relatively high production costs.
Mute testimony to the continuing lack of a fully satisfactory solution is provided by existing sludge handling practices at the above-mentioned Carson sewage treatment plant, which in part resembles what is being done at many other plants across the country. About half of the approximately 200 tons per day of sludge produced at Carson is still being trucked to landfills at a moisture level of about 80-85%, notwithstanding the above-described space and economic penalties associated with disposing of sludge in this manner. Much, if not all, of the remaining Carson sludge is sold as a soil conditioner, both in bulk and bags, operations which are benefitted significantly by drying the sludge. However, the technique currently being utilized to dry the sludge is to windrow it in the open air on approximately 40 to 50 acres of valuable land in a major industrial area, holding it there for a period of at least about a month with periodic turning over of the windrows, until it dries from its initial moisture content of 85% down to about 60%. Other communities with poorer weather conditions, lower levels of sludge production, and/or more capital to invest have turned to space- and time-consuming composting procedures in green houses and very large tanks.
For the above reasons and others, it is believed there continues to be a need for improved systems for the drying of sludge. It is the object of the present invention to satisfy this need.